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3 ELECTRICAL CONNECTIONS
Warning.
•
Although certain instruments are fitted with internal fuse protection, a suitably rated external protection device, e.g. fuse
or miniature circuit breaker (MCB), must also be fitted by the installer.
•
Before making any connections, ensure that the power supply, any high voltage-operated control circuits and high
common mode voltage are switched off.
3.1 Access to Terminals – Fig. 3.1
1
Release the lower panel and remove
from the front of the instrument.
Fig. 3.1 Access to the Terminal Block
3.2 Connections, General
Warning. The power supply earth (ground) must
be connected to ensure safety to personnel, reduction of
the effects of RFI and correct operation of the power
supply interference filter.
Information.
•
Earthing (grounding) – stud terminal(s) is fitted to the
transmitter case for bus-bar earth (ground) connection
– see Fig. 3.3.
•
Cable routing – always route the signal cable and
mains-carrying/relay cables separately, ideally in
earthed (grounded) metal conduit.
Ensure that the cables enter the transmitter through
the glands nearest the appropriate screw terminals
and are short and direct. Do not tuck excess cable into
the terminal compartment.
•
Cable glands & conduit fittings – ensure a moisture-
tight fit when using cable glands, conduit fittings and
blanking plugs/bungs (M20 holes). The M16 glands
ready-fitted to wall-mounted instruments accept cable
of between 4 and 7 mm diameter.
8
Information.
•
Relays –the relay contacts are voltage-free and must
be appropriately connected in series with the power
supply and the alarm/control device which they are to
actuate. Ensure that the contact rating is not
exceeded. Refer also to Section 3.2.1 for relay contact
protection details when the relays are to be used for
switching loads.
•
Current output – Do not exceed the maximum load
specification for the selected current retransmission
range – see SPECIFICATION, Section 7.
Since the current output is isolated the –ve terminal
must be connected to earth (ground) if connecting to
the isolated input of another device.
3.2.1 Relay Contact Protection and Interference
Suppression – Fig. 3.2
If the relays are used to switch loads on and off, the relay
contacts can become eroded due to arcing. Arcing also
generates radio frequency interference (RFI) which can result
in instrument malfunction and incorrect readings. To minimize
the effects of RFI, arc suppression components are required;
resistor/capacitor networks for AC applications or diodes for
DC applications. These components can be connected either
across the load or directly across the relay contacts. On 7320
instruments the RFI components must be fitted to the relay
terminal block along with the supply and load wires – see
Fig. 3.2.
A – A.C. Applications
L
External
A.C. Supply
B – D.C. Applications
External
+
D.C. Supply
Fig. 3.2 Relay Contact Protection
NC C NO
Relay Contacts
R
C
Load
N
NC C NO
Relay Contacts
Diode
Load
–
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